Channel coding is used to ensure the reliable transfer of information. In the absence of channel coding, as occurs with the CS-4 and MCS-4 GPRS/EGPRS coding schemes, the radio link performance is poor except under the most benign conditions. However, performance can be significantly improved by utilizing the CRC error detecting code in conjunction with the soft valued output of the demodulator for partial error correction.
In systems without error-correction coding, in the absence of channel decoding, a single estimated bit sequence is passed to the error-detecting decoder. The received frame is declared error-free if the CRC check succeeds. Prior-art methods dealing with improving the performance of non-coded systems rely on improving the demodulator. Usually these methods are computationally-intensive and the gain is marginal.
In the absence of channel coding, as is the case in the CS-4 coding scheme of GPRS, a single most-likely bit sequence is obtained from the demodulator (frequently, a Maximum Likelihood Sequence Estimator [MLSE] equalizer or other advanced receiver). This bit sequence is checked for error-detection using a CRC decoder. The received frame is declared error-free if the CRC check succeeds (i.e. the syndrome is zero). Otherwise, the frame is said to be in error.